The smartphone-driven capacity demand and the roll-out of 4G/long term evolution (LTE) networks requires service providers to deal with a far greater level of capacity demand than was anticipated in the days of 2G and 3G mobility. Backhaul has become a crucial enabler for mobile operators deploying new high capacity technologies like LTE. Its importance will increase further with the deployment of small cells and the associated increase in the number of cell sites, mostly in challenging urban locations. Given the shortage of fibre infrastructure in these areas, operators will need to invest in wireless backhaul links.

In rural areas, telecom requirements are being supported through microwave due to the low demand for telecom services. However, these areas have already started witnessing a rise in data uptake. As consumer demand evolves and mobile and internet penetration grows, operators will need to upgrade their backhaul networks.

However, this is easier said than done. Telecom operators face a double-edged dilemma as the increase in data traffic is not accompanied by an equivalent increase in revenue. According to an industry estimate, a 10-fold increase in data capacity only translates into a 20-30 per cent increase in revenues. This decoupling of revenues from capacity means that operators need to find and implement innovative solutions to manage backhaul costs to keep them in lockstep with this constrained revenue growth. As backhaul capacities transform from Mbps to Gbps, the technologies and solutions chosen to provide this capacity must be capable of delivering significant economies of scale.

The other key challenge is the limited availability of spectrum. Network vendors go to great lengths to maximise the aggregate throughput in the limited amount of wireless spectrum in their networks. As a result, there is a substantial need for cost-effective non-radio frequency solutions that can move backhaul traffic off of overworked wireless frequencies. Each current backhaul solution at present has pros and cons in its ability to address this spectrum crunch.

The other challenges for operators are related to frequency interference, non line of sight and larger hop distances. They also have to replace their legacy transmission equipment, which does not support IP. Therefore, they will have to undertake new transport planning for successful and cost-effective migration to an all-IP network. The IP-fication of both legacy and next-generation radio equipment is necessary to move to pure-IP backhaul play.

LTE-A poses further challenges

LTE-Advanced (LTE-A) Release 10 is a major enhancement of the LTE standard developed by the 3rd Generation Partnership Project (3GPP). This new technology is targeting peak data rates of up to 1 Gbps and introducing new concepts with the ultimate goal of designing a system that is significantly enhanced in both cell capacity and coverage. However, the deployment of LTE-A technology will add to the challenges of the current mobile backhaul network architecture. Inter-cell interference coordination (ICIC) and coordinated multi-point (CoMP) transmission are two functions from the LTE-A toolkit that target a better user experience at the cell edge. ICIC limits cross-talk by coordinating spectrum allocation across multiple cells, while CoMP allows multiple base stations to simultaneously serve a user device and boost the receive power level and capacity.

Both functions require very short latencies across the backhaul network to achieve real-time coordination between base stations. This means implementing the X2 interface as defined in the LTE and LTE-A standards. It facilitates direct communication between adjacent base stations. In order to meet the stringent latency requirements of less than 1 millisecond, the physical and logical path of the X2 interface needs to be as short as possible. Supporting this requirement is not trivial for most existing mobile backhaul deployments as the present underlying architecture is often designed according to a strict hub-and-spoke principle, where traffic distribution and redirection is architected in the distribution layer of the backhaul network.

In addition to low-latency connections, base station clocks need to be in phase to enable the proper operation of ICIC and CoMP. This leads to highly accurate phase or time-of-day synchronisation. Most 3GPP base station clocks are currently synchronised only in terms of frequency, since accurate phase synchronisation has not been a requirement until now. The new LTE-A functions, however, require base stations to be in phase with an accuracy of 500 ns for efficiently operating ICIC and CoMP. This is nearly impossible to achieve without on-path support, which means that the backhaul network needs to actively support timing distribution architecture.

Heterogeneous radio access networks (RANs) create further challenges. They are quickly becoming a reality, with RANs being composed of different types of base stations for maximising access capacities, optimising user experience and reducing costs. Base stations can differ in terms of capacity, reach, transmission power and RAN technology, including 3G, 4G and Wi-Fi.

LTE-A, along with tighter coordination between base stations, will, therefore, challenge existing backhaul networks in terms of capacity, latency and synchronisation performance. The current architectures will need to evolve for enabling mobile network operators to seamlessly migrate to LTE-A and enhance cell capacity, coverage and mobile user experience.